I'm absolutely blown away by this new project Luke Bayes and Ali Mills (of AsUnit fame) have just released! AsProject is a cross platform open source ActionScript development tool set that supports both ActionScript 2.0 and 3.0 and leverages tools like MTASC, Swfmill and AsUnit using rake tasks to automate the build process.
Its distributed over Ruby Gems, so the only thing you need to get started is Ruby and Ruby Gems installed -- everything else gets handled for you and you're literally doing open source actionscript development in minutes. This is really huge, open source Flash development wasn't rocket science before but this sure lowers the threshold for anyone still doubting to give it a try.
Best practices development at your finger tips, I love where the community is going with these open source initiatives!